Engines may operate with varying fuel blends. For example, vehicles may operate on a range of fuels supplied by the operator, ranging from pure gasoline to so-called E85 (an ethanol and gasoline blend with 85% ethanol). Various approaches are used by an engine controller to determine the fuel composition before engine operation. One approach to identifying the fuel make-up in a fuel tank is based on the shift in combustion stoichiometry caused by the varying fuel make-up. For example, at stoichiometry (e.g., as determined from the exhaust air-fuel ratio sensors), the amount of fuel delivered for the current amount of air is determined from injector characteristics. The stoichiometric ratio can then be estimated, thus enabling an ethanol content, for example, to be determined.
However, the inventors herein have recognized that while such an approach can be improved in various ways, it nevertheless remains highly susceptible to part-to-part variation and sensor drift. For example, changes in the injector characteristics can lead directly to errors in the ethanol estimate.
The above issues can be at least partially addressed by recognizing that the relationship between the fuel make-up and fuel evaporation on the engine intake ports, particularly during transient operation, can be used to infer the ethanol content of the fuel. This is because the ethanol content of the fuel affects how fuel evaporates from the puddles generated on the intake port during engine operation. Further, the effects of such fuel evaporation can be observed in the exhaust air-fuel ratio during transient operating conditions.
One example approach for determining the fuel composition includes adjusting engine operation in response to a fuel make-up, the fuel make-up based on variation in evaporation of fuel on an engine intake port. In this way, it is possible to more accurately account for the fuel make-up by reducing sensitivity to part-to-part variability and sensor drift. Further, the fuel make-up can be learned even during transient operation so that more accurate engine control can be provided during both transient and steady state operating conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.